linux-kernel - Re: [PATCH RFC] kvm: fast-path msi injection with irqfd

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Message-ID: <4CE4F823.8030804@redhat.com>
Date:	Thu, 18 Nov 2010 11:55:47 +0200
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Xiao Guangrong <xiaoguangrong@...fujitsu.com>,
	Gregory Haskins <ghaskins@...ell.com>,
	Chris Lalancette <clalance@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] kvm: fast-path msi injection with irqfd

On 11/18/2010 12:12 AM, Michael S. Tsirkin wrote:
> Store irq routing table pointer in the irqfd object,
> and use that to inject MSI directly without bouncing out to
> a kernel thread.
>
> While we touch this structure, rearrange irqfd fields to make fastpath
> better packed for better cache utilization.
>
> Some notes on the design:
> - Use pointer into the rt instead of copying an entry,
>    to make it possible to use rcu, thus side-stepping
>    locking complexities.  We also save some memory this way.
> - Old workqueue code is still used for level irqs.
>    I don't think we DTRT with level anyway, however,
>    it seems easier to keep the code around as
>    it has been thought through and debugged, and fix level later than
>    rip out and re-instate it later.
>
>

> @@ -166,6 +178,7 @@ irqfd_ptable_queue_proc(struct file *file, wait_queue_head_t *wqh,
>   static int
>   kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
>   {
> +	struct kvm_irq_routing_table *irq_rt;
>   	struct _irqfd *irqfd, *tmp;
>   	struct file *file = NULL;
>   	struct eventfd_ctx *eventfd = NULL;
> @@ -215,6 +228,10 @@ kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
>   		goto fail;
>   	}
>
> +	rcu_read_lock();
> +	irqfd_update(kvm, irqfd, rcu_dereference(kvm->irq_routing));
> +	rcu_read_unlock();

Wow, complicated.  rcu_read_lock() protects kvm->irq_routing, while 
we're in the update side of rcu-protected irqfd->irq_entry.

A comment please.

The rest looks good, it's nice we finally got the irq injection path so 
streamlined.

-- 
error compiling committee.c: too many arguments to function

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